Lab 2: GPC
1. Introduction
Gel permeation chromatography (GPC), more correctly termed size exclusion
chromatography, is a separation method, which is widely used for estimating the
molecular-weight distributions of polymers. The separation takes place in a
chromatographic column filled with gel beads that are rigid, highly porous and non-ionic.
Highly cross-linked porous polystyrene and porous glass are preferred column-packing
materials.1
In a typical GPC experiment, a dilute polymer solution is introduced into the
GPC column in which an appropriate solvent is flowing. A GPC column does not
separate according to mass, but rather via other mechanisms. As the dissolved
polymer molecules flow through the porous beads, they can diffuse into the internal pore
structure of the gel to an extent depending on their size and the pore size distribution of
the gel. The largest polymer molecules of the solute cannot penetrate the pores within the
cross-linked gel beads, and thus elute first (their retention volume is smaller). Smaller
polymer molecules of the solute are retained within the gel beads. The larger the
molecule, therefore, the less time it spends inside the gel, and the sooner it flows
through the column. The different molecular species are eluted (hence called elution
time) from the column in order of their molecular size as distinguished from their
molecular weight, the largest emerging first (Figure 2.1).2
Once the polymer molecules are separated and come out from column, at least
one detector is needed to tell what and how many polymer molecules are at a certain time
(elution time). The typical detectors are RI (Differential Refractive Index), UV
(ultraviolet) and LS (light scattering detector). Some not-so-common detectors can also
be found in the GPC systems, such as capillary viscometer detector
The most common 1) RI detector in use today is based on the deflection of a
beam of light as it passes through a dual compartment flow cell. One side of the
Fred W. Billmeyer, JR. “Textbook of Polymer Science”, Third edition, A willeyInterscience Publication.
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Jan F. Rabek, “Experimental Methods in Polymer Chemistry”, Physical Principles and
Applications, A Wiley-Interscience Publication.
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compartment contains the reference solvent of refractive index no, which is static during
the measurement process. The other side contains the sample solution, i.e., the column
eluent, having refractive index n. The differential signal will give the concentration of
eluent. In order to use the RI detector quantitatively, the calibration constant must
be determined. 2) UV detector is similar to a UV spectrometer, but only detects the UV
absorption at the certain wavelength. The absorption is proportional to the concentration
of eluent. 3) Light scattering detector is based on the fact that intensity of light scattered
by a polymer molecule is proportional to the square of its mass. LS can be used to give
the 'true' molecular weight data for polymer samples, additionally, the information on
differences in structure (or composition) between samples. Light scattering detector
works with at least one IR detector, which will give the concentration information that
LSD needed.
GPC is extremely valuable for both analytic and preparative work with a wide
variety of systems ranging from low to very high molecular weights. The method can be
applied to a wide variety of solvents and polymers depending on the types of gel used.
With polystyrene gels, relatively non-polar polymers can be measured in solvents such as
tetrahydrofuran, toluene, or (at high temperature) dichlorobenzene. With porous glass
gels, more polar systems, including aqueous solvents, can be used. A few milligram of
sample is sufficient for analytical work, which could be completed in as short as a few
minutes using modern high-pressure, high-speed equipment.
Porous Gel
Bead
Polymer
Molecule
Figure 2.1. Principle of the separation of molecules according to size by gel permeation
chromatography3
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ERS Department, Trent University,
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Earlier, it was said that GPC is used to estimate the molecular weight distribution
of polymers, including: weight average molecular weight (Mw), number average
molecular weight calculation (Mn), polydispersity index (PDI) and average degree of
polymerization ( DP ). Theoretically, this information could be obtained using the
formulae listed below as Equation 2.1 through Equation 2.4. In these equations, N refers
to the number of polymer chains having molecular weight M and for DP calculation, a
molecular weight of 104 Daltons for polystyrene will be used. These values are essential
in polymer characterization. Molecular weight (calculated as Mn or Mw) of a polymer,
for example, is related directly to a polymer’s physical properties. While Mn represents
the number average molecular weight, Mw represents the weight average molecular
weight. The ratio of the two gives information about the distribution of the chain lengths
within a polymer, a term known as polydispersity index (PDI). The average degree of
polymerization ( DP ), which is not as important as the other three parameters, represents
the average number of monomers that make up the polymer chains.
N M

N M
i
Mw
2
i
i
i
i
i
i
i
i
(Equation 2.1)
i
i
N M

N
i
Mn
w M

w
i
i
(Equation 2.2)
i
i
PDI 
DP 
Mw
Mn
Polymer molecular weight (M n )
Monomer molecular weight
(Equation 2.3)
(Equation 2.4)
http://whale.wheelock.edu/bwcontaminants/analysis.htmL.
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2. Experimental Procedure
IMPORTANT!!
Gloves MUST be worn when preparing the sample solution.
2.1 Sample Preparation
WARNING: Anytime you are handling your sample or any solvents you
MUST wear rubber gloves and have your hair tied back if it is long when preparing
the sample.
1. Prepare your polymer sample in the following way: weigh out 25 mg of polymer
3 (i.e., 400mg BPO PS sample synthesized in Lab 1) with the balance and then
add 5 mL of high performance liquid chromatography (HPLC) grade chloroform
so that the concentration is 5 mg/mL.
2. Filter the 5mL 5mg/mL PS chloroform solutions into a clean 2 mL vial by
drawing the mixture with a 5 mL extraction needle, removing the needle and
replacing it with a 0.45L filter and filling the 2mL vial about ¾ full.
2.2 Sample Loading
1. Open front panel: carousel access door (listen for motor to stop).
2. When motor has stopped, pull out carousel.
DO NOT RPLACE (or use) POSITIONS 90-95 ON CAROUSEL, THESE ARE
THE STANDARDS
3. Put sample vial in carousel and note location number
4. Return carousel and close door
2.3 Software Setup
1. Press the Sample Queue button on the left. The spreadsheet seen in the middle
right frame of Figure 2.2 will appear. Enter vial number and name in appropriate
columns.
2. In the Function column, click on the cell and select “Inject Broad Samples” from
the dropdown menu. In the Method column, click on the cell and select
“20120213_PS” from the options. Set your Run Time to 30 minutes. In the
Injection Volume (Inj. Vol.) column, type in a volume of “25”. In the Processing
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column (use the location bar at the bottom of the spread sheet to move to the right
of spreadsheet where this column is located) select “Don’t process or report”.
3. In the # of injections column enter the number “1”. Next injection delay should be
1 minute (DO NOT CHANGE). The data start column should also be 1 minute
(this is the delay before collecting data).
Figure 2.2: Breeze Software.
2.4 Sample Injection
1. Run current sample method (click “triple vial” button on lower left).
2. Name the sample same as before.
3. Run Mode: Run and Process. Run button (30 min. data)
4. The test will run for 30 minutes and the results will show on the right bottom
corner in real time.
5. When the data has been collected, you need to save your data on a portable drive.
Click on the Find data button and choose test file name, then select Test Profile
(right click) and choose: process (this will open a window). In the window, next
to Processing, check the box for process and choose “use specific method:
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20120203_PS”. Next to Exporting, check the export box and choose “use specific
export method: MSE453”. Click OK. This will send your data to the MSE 453
folder on the desktop. Your data is a .arw file, which can be opened with
MSexcel.
2.5 Removing the Sample
Open the front panel door (listen for motor to stop)
Remove carousel and retrieve sample
Return carousel and close door
3. Assignments
1). Based on the data obtained in the GPC experiment, and using a PS Standard
calibration curve data (Table 1), please perform the following calculations on the PS
sample: (1) weight average molecular weight (Mw), (2) number average molecular
weight (Mn), (3) polydispersity index (PDI) and (4) degree of polymerization ( DP ) (i.e.,
both Xw and Xn). Note: Please include detailed calculation based on the GPC curve in
your Lab Report.
2). Are there more than one peak in GPC curve? If so, explain why.
3). Based on the free radical polymerization kinetics, how should the concentration of
initiator affect molecular weight of PS? And what is the experimental observation on the
order of molecular weight of your PS? What values do you expect to find for the
molecular weight of the other two PS polymer samples synthesized?
Table 1-Monodispersed Standards Elution time
Elution Time (min)
14.033
14.51
14.778
14.99
15.679
17.525
18.125
20.004
20.85
22.692
24.406
25.376
MW (monodisperesed standard)
1260000
803000
432000
400000
178000
42000
30740
10800
6870
2600
1010
520
Log MW
6.100371
5.904716
5.635484
5.60206
5.25042
4.623249
4.487704
4.033424
3.836957
3.414973
3.004321
2.716003
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